The proposed research is designed to follow up on the considerable amount of evidence that 1) indicated that parafollicular cells of the mammalian thyroid gland contains both calcitonin and serotonin and 2) supports the hypotheses that thyroidal serotonin may act as a follicular cell activator. We propose continuing our investigation of the ultrastructural cytophysiology of thyroid parafollicular cells and also of thyroidal aminergic mechanisms. Many of the proposed studies will be carried out in the bat because its thyroid gland has been found to contain large numbers of parafollicular cells with seasonal synchronization of function. Other studies will be carried out in the sheep because the sheep's thyroid is large and is therefore an ideal model for some of the biochemical studies. Properties of thyroid aminergic mechanisms to be studied will include subcellular storage mechanisms for serotonin, tryptophan and serotonin uptake and its relation to thyroid activity, serotonin release and the role of serotonin in regulating follicular cell pseudopod formation. We also propose to continue our study of the role of the parafollicular cell during development employing an integrated approach in an attempt to determine if parafollicular cell secretion occurs prior to birth.